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Pulsars, Quasars, and Distant Questions

Introduction

In the 1960s, the discovery of two new phenomena, pulsars and quasars, sparked astrophysical research that continues to yield important results. Both are powered by collapsed ultradense objects and share some properties by virtue of their extreme nature; however, they are different phenomena. Pulsars are associated with the end point of the life-cycle of some stars, and quasars are associated with galactic centers.

Pulsars are rotating neutron stars, dense stellar cores left after a star implodes and then explodes during a catastrophic event known as a supernova. Their existence was predicted in 1932, the year the neutron was discovered, when Russian physicist Lev Landau (1908-1968) wondered if stars could contain “neutronic” matter deep within their cores. At the time no one knew how to detect a neutron star. They were later discovered in an experiment designed to detect radio galaxies, which were ultimately linked with quasars. Quasars are energetic powerhouses in the nuclei of galaxies. The most luminous objects in the universe, their discovery came as a complete surprise.

Though both were discovered by analyzing low-energy radio waves, they have since been studied with higher-energy x rays and gamma rays, giving astronomers a glimpse of violent phenomena in the universe. Recently, pulsars have allowed observational tests of general relativity and quasars are being used to map the distribution of matter filaments at the largest scale of cosmic structure.

Historical Background and Scientific Foundations

The Discovery of Pulsars

In 1967, Antony Hewish (1924-) and his graduate student Jocelyn Bell (now Jocelyn Bell Burnell, 1943-) built an array of radio antennae to survey over 1,000 radio galaxies. Unlike optical telescopes, which collect and focus light in visible wavelengths, radio telescopes tune in to long-wavelength electromagnetic waves—radio waves. Because Hewish was interested in rapid variations in the intensity of the sources, data from the array were recorded by movable pens on scrolling charts. One day’s observations produced almost 100 feet (30 m) of paper. At the time, any form of digital recording would have been prohibitively expensive.

A few weeks after beginning the survey, Bell noticed a bit of “scruff” on the charts that didn’t look either like a radio galaxy or local noise. The source appeared as a pulse that repeated precisely every 1.3373011 seconds. Further, each pulse lasted only 0.016 seconds, indicating that the source was compact. The distance was found to be 214 light-years, beyond the distance of the nearest stars, but still a typical stellar distance. After carrying out enough observations to rule out signaling from extraterrestrial civilizations (a time that Hewish later described as the most exciting in his life) he proposed that the source could be a white dwarf or neutron star undergoing periodic oscillation. These new objects were dubbedpulsars, for “pulsating radio sources.”

Early Theoretical Work on Neutron Stars

Much of the theory of white dwarf stars was developed in the 1920s by the Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910-1995), who applied the concepts of quantum mechanics to the interiors of stars. Once a star exhausts its nuclear fuel, he reasoned, it would contract under gravity until the Pauli exclusion principle caused electrons to exert a repulsive force. This force could support stars up to a maximum mass of 1.4 times the sun’s mass; for higher masses, the white dwarf would collapse to an even denser object.

Other theoreticians were also exploring the endpoints of stellar life cycles around this time. In 1934, astronomers Walter Baade (1893-1960) and Fritz Zwicky (1898-1974) predicted that the explosion of a massive star in a supernova would leave behind a remnant even denser than a white dwarf—a cluster of closely packed neutrons that they dubbed a neutron star.

In 1939, J. Robert Oppenheimer (1904-1967) and George Volkoff (1914-2000) published a detailed theoretical model indicating that neutron stars would have a mass greater than the sun’s, with a probable radius of only 6.2 miles (10 m). This tiny size implied two things:(1) neutron stars would have extremely high densities, approximately 1014 g/cm3, far greater than the sun’s average density of 1.4 g/cm3 and roughly equivalent to that of a neutron itself; and (2) they would be difficult to detect.

Connecting Pulsars and Neutron Stars

After Hewish, Bell, and three collaborators announced their pulsar discovery in February 1968, pulsars quickly became a popular research item. During the remaining 10 months of the year, more than 100 papers detailing new observations of pulsars or attempts to decipher the pulsar phenomenon were published. The correct interpretation was given by Franco Pacini (1939-), an Italian astrophysicist who was visiting Cornell, a few months before Hewish and Bell made their discovery.

Pacini predicted that rapidly rotating neutron stars with strong magnetic fields could accelerate electrons to the point that they emitted large amounts of beamed energy—the radio energy that Hewish and Bell detected. Two beams would shoot out of the star in opposite directions, one from each magnetic pole. As the star rotated, one of these beams might happen to sweep repeatedly over our own location, producing a pulsating signal like the blink of a rotating lighthouse light. Pacini’s model, known as the lighthouse model, predicted the sources must be ultracompact (by astronomical standards), otherwise the rapid rotation would tear the object apart.

In 1968, a year after Pacini’s work, Thomas Gold (1920-2004), another Cornell astronomer, independently proposed rotating neutron stars as the source of pulsar radiation. Ironically, Gold’s and Pacini’s offices were almost side by side. Only after their papers were published did the two scientists realize they had a common theoretical interest; they published a joint paper a month later.

Further discoveries that year helped pull the entire picture together. Pulsars were discovered in the Crab and Vela Nebulae, two clouds of gas known to be remnants of supernovae, verifying Baade and Zwicky’s 1934 theory. The Vela Pulsar had a period of 89 milliseconds, and the Crab Pulsar’s was even shorter, only 33 milliseconds, implying that these objects were spinning an astounding 11 and 30 times per second, respectively (in comparison, the sun spins about once a month, Earth once a day). Only neutron stars could spin that fast without being disrupted. Pacini and Gold had each predicted that pulsars would slow down over time, and the verification of this slight effect in the Crab Pulsar removed any lingering doubts that pulsars were rotating neutron stars.

The enormous densities of neutron stars help them keep extraordinarily accurate time. The Crab Pulsar is slowing by 36.5 nanoseconds per day. At this rate, it would take more than 75,000 years to lose a single second. However, a handful of pulsars have their periods disrupted for several weeks—in a disruption or “glitch”—before settling back into their old pattern of spindown. These are being studied to learn about neutron stars’ bizarre interiors, somewhat as geophysicists study vibrations from earthquakes to learn about Earth’s inner structure.

The Discovery of Binary Pulsars

Many pulsars discovered at radio wavelengths were subsequently discovered using optical telescopes. Long interested in observing wavelengths beyond optical or radio, in 1970, Riccardo Giacconi (1931-) and his colleagues used the x-ray satellite Uhuru (Swahili for “freedom”), to make two important discoveries: Pulsars can exist in binary systems, and they can emit x rays.

Among 300 x-ray sources detected by the satellite was the source Cen X-3, which was found to pulsate every 4.84 seconds, varying over a period of a few days. Instead of slowing down, however, this pulsar was found to be speeding up—it was gaining energy.

This surprising result occurred because the pulsar was part of a binary system, in orbit around a normal star. Astronomers had wondered if this type of pulsar existed, or if the force of the supernova explosion would permanently disrupt the pair. Here the pulsar was accreting matter from its companion, which caused it to spin faster. Because of a pulsar’s compact size, gas from the star was heated to high temperatures as it fell into orbit around the pulsar. When it fell onto the pulsar poles, it emitted x rays. Giacconi had found a new energy source for celestial objects: infall of accreting material. This is now thought to be the energy source of quasars.

As Hewish and Giacconi showed, surveys have always been good ways for astronomers to detect unusual objects. By 1974, about 100 pulsars had been discovered, and detection methods had improved for finding more. Russell Hulse (1950-) and Joseph Taylor (1941-), working at the University of Massachusetts at Amherst, were particularly interested in finding either very short-period or distant pulsars because they had been missed on previous surveys. To carry out such sensitive work, they used the Arecibo radar-radio telescope in Puerto Rico, the largest single dish radio telescope in the world. Over the course of 14 months, their survey netted 40 new pulsars, including what has become one of the most famous of all pulsars, PSR 1913+16 (PSR stands for pulsar and 1913+16 refer to the object’s sky coordinates).

When Hulse measured the period of PSR 1913+16 on July 2, 1974, he wrote “Fantastic!” in his notebook, because at 59 milliseconds, this was the second-fastest pulsar known, after the Crab Pulsar. However, strangely, the period was drifting to shorter values and then increasing. After collecting data for three weeks, Hulse had it figured out: the period was changing because the pulsar was in orbit around another equally compact object—he was observing a binary pulsar.

About half the stars in the galaxy are in binary (two-star) systems. By applying Newton’s laws to these orbits, one can determine their mass, a fundamental property. System PSR 1913+16, often referred to as the binary pulsar, was soon found to have remarkable properties. The two neutron stars (only one was observed as a pulsar) orbited each other with a period of 7ﬂ hours in a region smaller than the diameter of the sun. Each star had a mass of about 1.4 solar masses (1 solar mass equals the mass of our own sun) and orbital speeds were approximately 186 miles per second (300 km/s), ten times faster than Earth orbits the sun. Apart from accurately determining the neutron stars’ masses, the system as a whole provided a perfect laboratory to test various predictions made in Albert Einstein’s (1879-1955) theory of general relativity, published in 1916.

Testing Einstein

According to Einstein, large masses like stars will curve the space around themselves, and any sudden changes in their gravitational field will cause the emission of radiation known as gravitational waves. Because pulsars are such accurate timekeepers, astronomers could use the regularity of their pulses to test for relativistic effects, such as gravitational waves.

In 1978, after four years of monitoring the binary pulsar, Taylor announced at a conference that the orbital period of the binary pulsar system was decreasing—the two objects were losing energy and spiraling together. Not only did the observed amount agree with that predicted by general relativity, it provided indirect evidence for the existence of gravitational waves, which were carrying away the lost energy. For this work, Hulse and Taylor won the Nobel Prize for physics in 1983.

Additional surveys yielded new surprises. In February 2004, an international team of astronomers using the Australian Parkes radio telescope and the British Jodrell Bank Observatory announced the discovery of a double pulsar—a binary system in which both objects are pulsars. The two pulsars have periods of 23 milliseconds and 2.8 seconds and orbit each other every 2.4 hours, three times faster than the binary pulsar. Within months of discovery, using the tiny variations in the pulse periods the astronomers were able to detect that the orbit of the double pulsar is shrinking by 0.25 inches (7 mm) per day, providing additional evidence for gravitational waves.

Neutron Stars and Gamma Ray Bursts

This small decrease in the orbit will eventually cause the two stars to merge in a spectacularly energetic collision that will release enormous amounts of short-wavelength high-energy photons known as gamma rays. Since the late 1960s, about one gamma ray burst per day has been detected somewhere in the universe. Some bursts release as much energy in 10 seconds as the sun will release over the course of its whole 10-billion-year lifetime. They are known to come in two varieties, longer or shorter than two seconds.

In 1999, long bursts were identified with an energetic class of supernovae known as hypernovae. Short bursts’ origins remained elusive, however, because they occur so quickly, and gamma ray detectors have difficulty in localizing sources. Many bursts were found to have an x-ray and optical afterglow. After the 2005 launch of the High Energy Transient Explorer and Swift (two satellites dedicated to the study of gamma ray bursts) astronomers pinpointed the location of several bursts. They found the energies of these sources to be consistent with the merger of two neutron stars or of a neutron star and a black hole.

Pulsar research is proceeding rapidly. Over 1700 pulsars have been found; more than 80 are binary or millisecond pulsars, with periods as short as 1.4 milliseconds (implying the object rotates 714 times per second!). By timing pulsar signals, astronomers hope to understand how pulsars evolve and to model the interior structure of neutron stars. With new surveys, astronomers remain hopeful that a neutron star—black hole system—and direct evidence for gravitational waves—can be found. It seems likely that pulsars will continue to provide a fascinating window into high-energy astrophysics for years to come.

The Discovery of Quasars

While theoretical predictions preceded observations for pulsars and neutron stars, quasars required a broad range of observation before any theoretical understanding was acquired. In 1960, Thomas Matthews and Allan Sandage (1926-) at Caltech set about finding optical counterparts for some of the intriguing objects in the recently completed Third Cambridge Catalogue of Radio Sources (3C).

Because individual radio telescopes have poorer resolution than optical telescopes of the same size, this was like using a rough geographical contour map to identify visible landmarks on a photograph. With the rough coordinates for a stellar object known as 3C48, Sandage used the 200-inch Hale telescope at Palomar Observatory in California (at that time the largest in the world), to take a photograph and a spectrum. Though the object looked like a star, its spectrum was more like that of a galaxy.

Sandage presented his results at a conference in December of that year, favoring the explanation that the object was a “relatively nearby star with most peculiar properties.” Object 3C48 and others like it soon became known as “quasi-stellar radio sources,” or quasars. Because Sandage later found that most quasars didn’t have strong radio emission, the term “quasi-stellar object,” or QSO, was also used.

Jesse Greenstein (1909-2002), a senior astronomer at Caltech and expert on stellar spectroscopy, also puzzled over the spectrum with Maarten Schmidt (1929-), a young Dutch astronomer new to Caltech. Though they wondered if the spectral lines could be those of familiar elements like hydrogen and helium shifted to longer wavelengths, they dismissed the idea as “wild speculation.”

In 1963, Cyril Hazard and his Australian colleagues pinpointed an accurate position for 3C273 by observing it when the moon, whose position is known precisely, passed in front of it. With the new coordinates, Schmidt photographed 3C273 and took its spectrum. Like 3C48, 3C273 appeared starlike, but with a wispy jet off to one side. Schmidt later wrote that its bizarre spectrum had “a rich variety of [emission] lines that made no sense to me.” But perhaps remembering his speculation with Greenstein, Schmidt soon detected the series of well-known hydrogen lines (from the Balmer series) displaced by 16 percent toward the red, or long-wavelength end of the spectrum. When he was told, Greenstein replied, “Impossible!” But it was true; Schmidt had cracked the quasar code.

The two men then reexamined 3C48’s spectrum and found it made sense if they shifted the hydrogen lines from their normal position by 37 percent. According to Hubble’s law, shifts in the spectra of distant galaxies are proportional to their distances: the farther away the galaxy, the greater the spectral shift (because farther galaxies are receding at greater velocities). Applying this to 3C48 meant that, rather than being a star in our galaxy, the quasar was 4 billion light-years away. The astronomers’ concept of the size of the universe increased suddenly and dramatically.

Schmidt’s identification of 3C273’s spectrum galvanized astronomers to find other distant quasars. For the next several years, research journals filled with papers announcing the new farthest thing in the universe. By 1973, 200 quasars had been found, with the farthest having emission lines shifted by more than 300% (indicating that the universe had expanded by this amount since the light was emitted)—it was rushing away at more than 90% of the speed of light.

Schmidt also realized that quasars were much more common at large distances than they were nearby. Because light takes so long to travel the enormous distances of the universe, seeing an object at a large distance is equivalent to seeing back in time, when the universe was much younger. The implication was that early in the history of the universe, quasars were much more common than they were at later times. In Timothy Ferris’s book The Red Limit, Schmidt described the situation: “Apparently quasars were more numerous in the early universe. They evolved, died, and now they are very rare. Probably they evolved into something else. I think they probably are the nuclei of galaxies, but mind you, that’s speculation.”

For several years in the late 1960s and early 1970s, the large distances implied by the measured quasar red shifts sparked a controversy. At issue was the true brightness of quasars. If they were as far away as the red shifts indicated, then, to be shining so brightly they had to pour out 10 to 100 times more energy than a normal galaxy containing 10 billion stars.

Evidence for the so-called cosmological distances of quasars came trickling in. In 1971, James Gunn (1938-) showed that a pair of galaxy clusters that contained quasars had the same red shifts as the quasars themselves. Two years later, Jerome Kristian (1934-1996) used the 200-inch Hale telescope to photograph 30 low—red shift quasars. He found that a significant fraction of them had a slightly fuzzy appearance, indicating, as Schmidt had speculated, that the quasars were surrounded by a galaxy of stars too faint to be resolved.

An additional puzzle was that quasars’ light outputs were found to vary on time scales as short as a few weeks or months. Because no source can turn itself on and off in the time it takes light to travel across it, the implication was that enormous amounts of radiation were coming from an ultracompact region only a few light-weeks or light-months across, much smaller than the distance between the sun and the nearest star, which is four light-years.

The Quasar Engine

The discovery of quasars coincided with a renaissance in testing Einstein’s theory of general relativity. Quasars’ extraordinary energy output demanded an extraordinary source, and several theoreticians posited that this was the strangest collapsed object of all: a black hole. Not long after laying the foundational work for neutron stars in 1939, Oppenheimer, with Hartland Snyder (1913-1962), used Einstein’s theory to model what would happen when a massive star exhausted its nuclear fuel. If it were too massive to become a white dwarf or a neutron star, the object would continue contracting until it became so dense that even light rays couldn’t escape from it. These objects were dubbed black holes.

In 1964, Russian physicist Yakov Zel’dovich (1914-1987) and American astronomer Edwin Salpeter (1924-), independently proposed that a black hole could be a powerful energy source by accreting gas from nearby stars. Because the gas would spin around the black hole in a high-velocity orbit, it would be heated to high temperatures, causing it to radiate. Friction would cause some of the gas to fall into the black hole, emitting high-energy radiation before it disappeared.

This model of gravitational infall was calculated to be a much more efficient energy-generating process than nuclear fusion in stars; even a few solar masses of infall material per year could give the required observed energies. (Observational evidence for high energies produced by gravitational infall was obtained by Giacconi, in the context of x-ray binaries, described above.) A few years later, in 1969, Donald Lynden-Bell (1935-) argued that because quasars were so much more luminous than galaxies, the black hole engine had to be massive, perhaps 10 million to 1 billion solar masses. He further suggested that such black holes wouldn’t have an unlimited fuel supply and that dead quasars could be common in nearby galaxies. For example, consuming two solar masses of material per year for a billion years would be the equivalent mass of an entire galaxy.

Observational Evidence for Supermassive Black Holes

Though the model appeared plausible, astronomers still wanted to find evidence that supermassive black holes truly did reside in the centers of galaxies. Because the giant elliptical galaxy M87 was known to have an optical jet streaming out of its center, it was one of the early targets for study by the refurbished Hubble Space Telescope (HST). In 1994, HST cameras revealed a thin disk of gas about 500 light-years in diameter at the core of M87. Spectra showed that at only 60 light-years from the center of the disk the gas whirled at a speed of 342 mi per second (550 km/s). By applying Kepler’s laws to the orbit, the team of astronomers, led by Holland Ford (1940-), calculated that the mass inside this volume had to be about 2.5 billion solar masses—a number much too large to be explained by ordinary stars. Since then astronomers have used the HST and other telescopes and found that, by peering into centers of galaxies, evidence for supermassive black holes exists in about three dozen galaxies so far.

Even our own Milky Way galaxy apparently has a black hole at its center. Various groups of astronomers have measured stars orbiting only a few light-days away from the radio source at the center of our galaxy, Sgr A* (pronounced “Sagittarius A star”), with speeds exceeding 621 mi per second (1,000 km/s). In 2002 the Chandra X-Ray Observatory at Harvard found huge lobes of hot gas (36,032°F/20,000°C) extending over dozens of light-years on either side of Sgr A*. Three years later, in 2005, Zhiqiang Shen of the Shanghai Astronomical Observatory and his colleagues used an array of 10 radio telescopes to infer that Sgr A* must be 4 million times more massive than the sun, but that it must also be smaller than the size of Earth’s orbit. Sgr A* is now among the best supermassive black hole candidates.

This evidence, together with the above HST census, indicates that black holes could be standard equipment in all galaxies. In addition, the evidence from our galaxy also suggests Donald Lynden-Bell might have been correct when he said that dead quasars could lurk in the centers of most nearby galaxies. If so, that leaves a puzzle: Why don’t they produce as much energy as their high—red shift relatives? Perhaps the black hole is not being fed, but astronomers would like to understand the process better because once an accretion disk forms, it is unlikely to dissipate.

The Quasar Era

One way to bring a fresh supply of gas and stars to feed a galaxy’s central black hole is through a collision with another galaxy. Indeed, a 1996 HST survey led by John Bahcall (1934-2005) searching for quasars 1 to 2 billion light-years away found that most of the galaxies that host quasars were either undergoing a collision with another galaxy or showed distortions in their shape that indicated a collision had recently occurred. Mergers are thought to play a role in galaxy formation as protogalactic gas clouds collide to form larger galaxies, but more data are needed for quasars at much larger distances in order to improve the picture.

Quasars are the most energetic objects in the universe, and they thrived when the universe was 10-20% of its present age of 13.7 billion years. Their enormous brightnesses allow astronomers not only to observe conditions that were present in the early universe but also the intervening matter illuminated by the quasar’s light.

In the 1990s, astronomer James Gunn conceived of the Sloan Digital Sky Survey, an ambitious sky-mapping effort. The survey’s quasar-hunting phase ran from 2000 to 2005, during which time spectra of about 675,000 galaxies, 185,000 stars, and 90,000 quasars were collected. This enormous database helps astronomers examine the era when quasars first began, and particularly to decipher whether they preceded or followed the formation of their host galaxies or whether the two somehow evolved jointly. The data will also help astronomers as they devise better models to explain how the quasars’ supermassive black holes formed, how jets form, and why the quasar era lasted about a billion years and then came to an end. In astronomy, the era of quasar discovery began just over 40 years ago, but while much progress has been made, much remains to be done before the quasar puzzle is completely solved.

Modern Cultural Connections

Growing scientific knowledge of quasars and pulsars has had little direct impact on the lives of most persons, who have never even heard of such objects. However, this knowledge has been crucial to the expansion of modern astronomy’s understanding of the structure of the cosmos and the nature of fundamental physical laws. As noted earlier, pulsars have allowed the experimental testing of general relativity, still physics’s best theory of gravitation. Measurements of pulsar signals from pulsars in binary systems have allowed detection of orbital precession, spiraling-in, gravitational redshift (reddening of light traveling against a gravitational field), and other phenomena predicted by general relativity, confirming general relativity’s predictions to within .05%. General relativity, in turn, has numerous practical applications; for example, the global positioning system (GPS) must take general-relativity effects into account in order to provide accurate position data to ground receivers.

Quasars, too, have become not only objects of study but tools for investigating other phenomena. In the late 1990s and early 2000s, astronomers realized that on a scale of the whole cosmos, matter (both the invisible, still-mysterious material dubbed “dark matter” and the visible gas, dust, and shining stars of ordinary matter) is distributed in a three-dimensional network of random filaments, somewhat like a ball of steel wool only less sharply defined. Mapping this filamentary structure has become a new goal of astronomy and cosmology, and quasars an essential tool in that effort. Hydrogen gas is located in and near the filaments, and light from quasars more distant than the gas is modified by passing through it. When quasar light passes through interstellar atomic hydrogen, some is absorbed, making a dip in its spectrum. The spectral location of this dip reveals the hydrogen’s recession velocity (and thus its distance); also, the deeper the dip, the more quasar light has been absorbed, indicating that it has passed through more gas. Analyzing the spectra of thousands of quasars will allow astronomers to map the filamentary matter structure of the universe. This, in turn, will allow certain tests of competing theories of the universe’s origin and ultimate fate.

Primary Source Connection

The following article was written by Peter N. Spotts, a science and technology writer for the Christian Science Monitor.Founded in 1908, the Christian Science Monitor is an international newspaper based in Boston, Massachusetts. The article describes science’s ongoing quest to understand dark energy, the mysterious cosmic force that makes up almost three quarters of the universe.

ASTRONOMERS AIM TO SHINE LIGHT ON UNIVERSE’S “DARK ENERGY”

It’s almost three-quarters of what the universe is made up of—but scientists are still trying to figure out what it is.

In nearly a decade since it was discovered, a mysterious cosmic feature dubbed “dark energy” has lain like a downed redwood across the path of scientists trying to reach the holy grail of physics—a fundamental theory of matter and its basic forces.

Unlike gravity, which binds galaxies together and gathers them into large clusters, dark energy would drive them apart. For several billion years, gravity acted as a brake, slowing a universe ballooning since it formed some 13.7 billion years ago in what cosmologists call the “big bang.” These days, however, dark energy appears to be taking over and speeding that expansion.

So far, no one has devised a widely accepted reason why dark energy exists. Nor has anyone figured out why it acts as a repellent. Yet cosmologists now calculate that dark energy is 74 percent of the universe’s inventory of matter and energy.

“It’s almost unfair that the universe is teasing us in this way. It gives us this dramatic clue, then shuts up,” says Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena. “We want to understand this dramatic fact much better. But in order to do that, we need to get more information about it.”

Late last week, a National Research Council (NRC) panel became the latest group to answer that call. It recommended that the National Aeronautics and Space Administration and the US Department of Energy underwrite a mission to take dark energy’s measure. After examining a range of possible NASA missions grouped under the heading “Beyond Einstein,” the panel concluded that efforts to probe dark energy had the right mix of critical science questions, available technologies, and reasonable cost.

If NASA proceeds as the NRC recommends, it would join attempts by other researchers using ground-based telescopes to find the missing pieces that would help solve the dark-energy puzzle.

Dark energy was discovered in 1998 by two groups working independently. They found that the universe was expanding faster than it should be, given the density of matter and energy the universe was estimated to contain.

Researchers spent the first six months or so after the discovery trying to answer the question, “Is this right?” says Adam Riess, an astronomer at Johns Hopkins University in Baltimore and a member of one of two teams. Among other things, their observations led them to estimate that the vast majority of the universe’s inventory of matter and energy was this befuddling dark energy.

Since then, Dr. Riess continues, researchers have used various approaches to confirm the discovery and to pin down the point in the universe’s history where, over very large distances, gravity began to yield to dark energy.

Four years ago, for instance, scientists using ground-based telescopes and a satellite to study the microwave “hiss” left over from the big bang gave the discovery a major boost. The satellite—the Wilkinson Microwave Anisotropy Probe—measured hot spots and cold spots in this background radiation, which represents the universe when it was 300,000 years old. The hot spots/cold spots correspond to different densities of matter across the sky. That pattern matched with the large-scale structure of the universe astronomers see today. It also confirmed that dark energy accounted for 74 percent of the universe’s total “matter-energy density.” Another approach uses light from exploding stars, or supernovae, at different distances as a kind of cosmic speedometer to track the universe’s expansion rate at different points in its history. One particular kind of supernova, a type 1A, provides the information. Its pattern of brightening and dimming acts as a fingerprint, allowing scientists to identify it. The pattern also tells astronomers how intrinsically bright it is.

Researchers then compare the supernova’s intrinsic brightness with its reduced brightness as viewed from Earth. This yields an estimate of the distance to the supernova. Then they turn to apparent changes in the light’s color as a measure of the universe’s expansion rate. The redder the light appears, the faster the supernova is speeding away—and the faster the universe is expanding at that distance.

Three years ago, Riess and colleagues used the Hubble Space Telescope to discover 16 supernovae, including six of the most distant ever seen. They added those to 170 previously analyzed supernovae, then looked for trends in deceleration and acceleration. They found that for the first 7.7 billion to 8.7 billion years of its life, the universe was expanding, but at an ever-slowing pace—just as the reigning theory of the universe’s birth and evolution predicted.

After that, however, the expansion rate began to quicken. So if the rate of increase in dark energy’s influence remains constant, anything beyond about 10 billion light-years away becomes an intergalactic “Shane,” riding into the sunset, never to be seen again. It vanishes because space beyond that distance would be expanding faster than the speed of light. If the impact of dark energy rises with time, the ultimate end could come with “the big rip,” tearing apart everything from galaxies to atoms.

Back in the real world, though, astronomers are happy with the challenges dark energy poses today.

“After about 10 years it’s clear [dark energy] is not going away. We have to really figure out what this is,” Riess says. The past decade also has shown that “dark energy lives at the crossroads of two of our best theories of physics: quantum mechanics and general relativity.”

A successful marriage of quantum theory and gravity is the last major hurdle in demonstrating that the basic four forces of nature—gravity, electromagnetism, and weak and strong forces that operate at the subatomic level—are manifestations of a single force that dominated the universe in the first few fractions of a second after the big bang. With dark energy, “nature is giving us a hint of how it does quantum gravity,” Riess says.

More observation is needed. The type of project the NRC recommends includes three candidates, all of them variations on a space-based telescope.

For example, a team led by Saul Perlmutter at the Lawrence Berkeley National Laboratory, proposes an orbiting observatory to view 2,000 supernovae a year. The hope is to turn the current jerky flip-book sequence of how dark energy’s influence changes with time into a smoother reconstruction to help determine the constancy of dark energy’s influence. It would also analyze how the distribution of matter changes with time, using another Einstein phenomenon: gravity’s ability to bend light when it passes by massive objects such as stars or galaxies.

“This tells you about the fight between dark energy and gravity” over time, says Dr. Perlmutter, who led the other team credited with discovering dark energy. If the supernova data and the light-bending data tell the same story about the expansion history of the universe, “then what you’re seeing is something like a dark energy.” If not, “then what you’re seeing is some modification of Einstein’s theory of gravity.”